1. Field of the Invention
The present invention relates to a thin film transistor substrate for use in a liquid crystal display device and a method of fabricating the same.
2. Description of the Related Art
A liquid crystal display device has advantages that it is thin and lightweight and that it can be driven at low voltage and low current consumption. In recent years, it is widely used as a display device for a personal computer and for a television receiver, etc.
Generally, a liquid crystal display panel of the liquid crystal display device is configured in which liquid crystals are sealed between two transparent glass substrates. On one surface of the surfaces facing each other of the two glass substrates, a black matrix, a color filter, a opposite electrode, an alignment layer, etc. are formed, whereas on the other surface, thin film transistors, pixel electrodes and an alignment layer are formed.
On the surfaces other than the surfaces facing to each other, a polarizer is attached. When the polarization axes of the two polarizers are orthogonalized to each other, a liquid crystal display device in the normally white mode is configured. More specifically, light is transmitted in the state in which no electric field is applied to liquid crystals, whereas light is blocked when an electric field is applied to liquid crystals. On the other hand, when the polarization axes of the two polarizers are disposed in parallel with each other, a liquid crystal display device in the normally black mode is configured. More specifically, light is blocked in the state in which no electric field is applied to liquid crystals, whereas light is transmitted when an electric field is applied to liquid crystals.
A liquid crystal display device before will be described with reference to FIGS. 11 to 13. FIG. 11 is a plan view illustrating a inverted staggered active matrix thin film transistor (TFT) substrate, and FIGS. 12 and 13 are cross sections of a dashed line A-A′ shown in FIG. 11. As shown in FIGS. 11 to 13, on a glass substrate 3, a gate electrode 27 formed of an Al film 50, an MoN film 54 and an Mo film 53 is formed. The gate electrode 27 is connected to a gate bus line 6 formed of the same conductive films.
The reason why Al is used as a material for the gate electrode 27 is that Al is a material having a low electric resistance. In the conventional liquid crystal display device, a high melting point metal having a relatively high electric resistance such as Cr is used for the gate electrode material, but in recent years, a low resistance material such as Al is used in order to cope with an increase in size and high definition of the liquid crystal display device.
The reason why the MoN film 54 and the Mo film 53 are formed on the Al film 50 is that Mo is a material that has a high heat resistance property and allows the Al film 50 to have an excellent electric contact in connecting it to other wirings. The gate bus line 6 is connected to a predetermined driver IC through ITO (Indium Tin Oxide) in an area not shown, and it can have an excellent electric contact because it is connected to other wirings through the Mo film 53. In addition, the Al film 50 tends to cause a hillock due to heating in the post-process to deteriorate the withstand voltage of a gate insulating film 32 that is contacted with the Al wiring. In order to prevent this event, a structure is provided in which a high melting point metal such as Mo is used for a cap.
On the glass substrate 3 formed with the gate electrode 27, the gate insulating film 32 is formed. On the gate insulating film 32, an amorphous silicon film 34 is formed. On the amorphous silicon film 34, an n+-amorphous silicon film 36 is formed. On the n+-amorphous silicon film 36, a source electrode 28 and a drain electrode 63 (see FIG. 11) formed of an MoN film 64, an Al film 65 and an MoN film 66 are formed. The drain electrode 63 also serves as a data bus line 8.
On a layer above the source electrode 28 and the drain electrode 63, a protective film 42 is formed. As shown in FIG. 11, the protective film 42 is formed with a contact hole 46 which reaches a pixel electrode 37. On the protective film 42, the pixel electrode 37 formed of ITO connected through the contact hole 46 is formed. Since the Al film 65 is connected to the pixel electrode 37 through the MoN film 66, it has an excellent electric contact. As described above, since Al of a low resistance metal is used for materials of the gate bus line 6 and the data bus line 8 in the liquid crystal display device, it can contribute to an increase in size and high definition of the liquid crystal display device.
Patent Reference 1: JP-A-2000-208773
Patent Reference 2: JP-A-2001-223365